IETF MMUSIC Working Group thanks 3GPP TSG SA WG4 for informing about their work
in <https://datatracker.ietf.org/liaison/1442/>.
It is understandable that SDP attribute-based approaches, such as solutions D
and F in 3GPP TR 26.924, are being considered for further development. Of the
alternatives proposed, use of an SDP attribute (i.e., an "a=" line) is to be
preferred over use of further extensions to SDP "b=" lines. Attributes are the
natural extension point for SDP, and offer flexibility that is not present in
"b=" lines. This is compatible with the solutions proposed by 3GPP TR 26.924.
Whether any new attribute is signalled per RTP payload type, as in solution D,
or per "m=" line, as in solution F, is a more complex issue. Neither seems
unreasonable on the surface, and a QoS attribute per "m=" line is natural if
each "m=" line represents a single media sent on a single port, with the SDP
offer/answer exchange being used only to select what encoding is used for that
media.
However, with the definition and widespread adoption of the SDP BUNDLE
extension
<https://datatracker.ietf.org/doc/draft-ietf-mmusic-sdp-bundle-negotiation/>,
the assumption that a single "m=" line represents a single type of media being
sent on a single UDP port is becoming less tenable.
IETF MMUSIC urges 3GPP TSG SA4 to consider the impact of the SDP BUNDLE
extension on their choice, and to further coordinate with IETF to ensure that
whatever is developed is compatible with SDP sessions where multiple "m="
lines, representing several different types of media, are sent on a single UDP
port.
Next IETF meetings:
- IETF 96, July 17-22, 2016, Berlin, Germany
- IETF 97, November 13-18, 2016, Seoul, South Korea
Regards,
Bo Burman
MMUSIC WG co-chair